The invention relates to variable optical attenuators, and in particular to optical switches providing an attenuation capability. The invention finds application to micro-electromechanical systems (MEMS), but it is not so limited.
The all-optical optical switch promises to become a key element of fiber-optic networks. Steering elements in the optical switch are used to direct beams of light from input fibers to the desired output fibers. The amount of optical power coupled through a given connection is a function of a) the amount of power present in the input fiber, b) insertion losses at the fibers, lenses, and mirrors, c) coupling loss due to mode radii mismatches and longitudinal errors, and d) coupling loss due to errors in beam alignment as effected by the steering elements.
Previous optical switches have not capitalized on the loss due to errors in beam alignment as a means of intentionally attenuating the power emerging from the output fibers en route to certain other elements of the network. One example of the need for and use of intentional power attenuation is the case of long-haul networks using Dense Wavelength Division Multiplexing (DWDM). An array of Variable Optical Attenuator (VOA) elements immediately upstream of a DWDM system attenuates the power in the incoming fibers to a uniform level as required by the DWDM. In a network employing optical switches in conjunction with power-level-sensitive elements such as DWDM systems, the attenuation function currently executed using dedicated VOA elements could potentially be absorbed by the optical switch, thereby minimizing or eliminating the need for expensive post-switch attenuators.
Variable attenuators are known that use intentional errors in free-space beam alignments as a means of attenuating fiber power. Two such attenuators are described in U.S. Pat. No. 5,915,063 and U.S. Pat. No. 6,137,941, assigned to Lucent Technologies. In these attenuators, a single mirror actuated with at least one actuator is used as a beam-steering element. Optical loss is governed by deflecting the mirror in at least one axis away from a set of angles that maximize coupling. Although this power-loss mechanism is similar to the loss mechanism employed in the present patent, the art of these patents is not applicable to the general case of using two or more steering elements, or to the specific case of an in-operation optical switch utilizing prescribed motions of the steering elements that prohibit the methods of the previous patents.
What is needed is a technique for optical attenuation that can be effected using the steering elements of an in-operation optical switch without deleterious impact on the switching functions.
According, to the invention, a method and apparatus are provided for the closed loop attenuation of optical beam power in a multiple-axis free-space-coupled single-mode fiber-optic transmission system, such as an optical switch. In a specific embodiment, the beans steering elements are two tip-tilt mirrors operative to couple optical power from an input fiber to an output fiber. The plurality of steering elements control both angle and translation of the optical beam at the output relative to a nominal optical axis. The four mirror axes are actuated in such a way as to produce either a static or time-varying set of induced mirror ankles that yield a desired time history of optical loss. The attenuation technique uses the DC level of the measured output power to adjust the amplitude of the induced mirror angles.
Since the attenuation technique is concerned with the DC level of measured power, the technique can be used simultaneously with compatible alignment-detection techniques that ignore the DC component and infer optical alignment through observation of signals at the frequencies of induced motions (dithers). A suitable alignment detection technique is described in a patent application Ser. No. 09/935,429 in the name of the present inventor entitled METHOD AND APPARATUS FOR OPTICAL BEAM ALIGNMENT DETECTION AND CONTROL.
The theoretical basis as presented here for four-axis variable attenuation is sufficient for the general case. Therefore, the disclosure is to be understood to address tile cases for applications of more or fewer than four axes with a plurality of steering elements in the optical path.
The invention will be better understood by reference to the following detailed description in connection with the accompanying embodiments.